The epidemiology of breast cancer is complicated and only partially understood, despite an enormous research activity in this area. It has been hypothesized that intrauterine factors and perinatal events or conditions could affect adult breast cancer risk in the female offspring. The factor that appears to be the most informative perinatal factor is birth weight; however, the studies exploring this potential predictor have reached different conclusions. The primary goal of the proposed study is to examine the hypothesis that perinatal and intrauterine factors will influence the female fetus' future risk of breast cancer. To this end, the following specific aims will be addressed: to investigate if there is a positive association between cancer risk and indicators of fetal size; to confirm the negative association between breast cancer risk and maternal eclampsial pre-eclampsia; to examine the observed positive association between breast cancer risk and prematurity and neonatal jaundice; and, to study the effect of other indicators of intrauterine environment, such as maternal overweight, parity, and age at time of birth as well as placenta characteristics, on the occurrence of breast cancer in the female offspring. To examine these relationships, a matched, nested case-control study of perinatal factors associated with breast cancer will be conducted among women in Norway. The population base for this study will be all female residents of Trondheim and Bergen who were born at the University hospitals in those two counties between 1930 and 1970. Incident breast cancer cases will be identified from the national Norwegian Cancer Registry, which has been in existence since 1952. Using the unique personal identification assigned to all Norwegians to obtain information on birthplace and mother's name, linkage would then be made to the case's birth record at the University hospitals. For each case, three age- and residence-matched control women who were born in the same birthing clinic as the case will be selected. It is anticipated that 1,000 new cases of breast cancer will have occurred through 1997 among the women born and residing in Trondheim and Bergen; thus, 3,000 matched controls would also be included in the study. Exposure information concerning the perinatal factors of interest will be obtained from the birth records. Access to data of such superb quality will provide an important opportunity to critically evaluate the intrauterine origin of breast cancer hypothesis.